The present invention relates to the field of pressure sensors, and more particularly to the field of diaphragm sensors using silicon-on-insulator (SOI) technology.
The use of pressure sensors or transducers has found numerous applications in a variety of fields in which it is desirable to monitor fluid flow, liquid level or pressure. The electronics industry has attempted to accommodate the need for low-cost and dependable sensors by utilizing integrated circuit (IC) fabrication techniques in the design and manufacture of microfabricated diaphragm transducers on IC chips. The basic types of silicon pressure transducers include (1) piezo-junction devices, (2) piezoresistive sensors, and (3) capacitive pressure transducers.
Piezoresistive sensors have been formed in polycrystalline silicon, laser recrystallized silicon and bulk single crystal silicon. The resistors have been fabricated by dopant implantation followed by annealing and metallization. The diaphragm of the sensor has been separately formed by patterning a silicon wafer with an oxide insulator, depositing polycrystalline silicon over the insulator/wafer surface and removing the oxide from between the diaphragm and the wafer.
A microfabricated capacitive pressure transducer is formed, for example, by diffusing a dopant into a region in a silicon wafer that serves as a lower electrode, and forming a compliant diaphragm of polysilicon as a second electrode that is separated from the diffused region by an oxide spacer. The oxide spacer can be removed by etching through an opening in the backside of the wafer. See R. S. Hijab and R. S. Muller, "Micromechanical Thin-Film Cavity Structures For Low Pressure and Acoustic Transducer Applications," IEEE, CH 2127, 178, September, 1985.